The present invention concerns a process for the industrial synthesis of urea, making ammonia and carbon dioxide react in at least one reaction space, at high pressure and temperature, and recycling at least partially the unreacted products.
The invention concerns also the applications of this process to conventional systems in order to improve the urea yield and reduce energy consumptions.
New or modified plants obtained by the implementations of the above mentioned process and its applications to pre-existing systems, are within the scope of the invention.
Known are several types of processes, systems and plants for the industrial synthesis of urea from NH3 (in excess) and CO2, with recycling after stripping.
In order to have immediately clear ideas referring to emblematic cases, one among the most stated processes is the one based on the technology (Snamprogetti) of isobaric stripping with the synthesis reactor (stripping with NH3, selfstripping), (FIG. 1).
In the stripper (S) a great part of the carbamate included in the urea solution coming out of the reactor (R) and part of the free NH3 present are stripped and recycled into the reactor, whilst a urea solution (SU) coming from the stripper (S) having a relatively low content of residual CO2 (5+7% weight) and obtaining a relatively high content of NH3 (22+25% weight). This solution (SU) is treated in a middle pressure phase (SMP) where is it distilled at 18+20 bar and the obtained vapours are sent to a rectification column (CR) enabling to obtain, first, NH3 at high purity (NEP) and then carbamate solution (SC).
The NH3 at high purity, after condensation, joins the NH3 feed (NA) and is pumped (pump P) into the reactor (R), and the same thing happens with the carbamate solution (pump P').
The main technical characteristics of the isobaric stripping process (Snamprogetti) can be resumed as follows:
synthesis pressure: approx. 150 bar PA1 NH3/CO2 mol in the reactor: approx. 3.2+3.4 PA1 H2O/CO2 mol in the reactor: approx. 0.6+0.7 PA1 temperature of the reactor: 190.degree. C. PA1 yield: approx. 62+63% PA1 vapour consumption: approx. 900 kg/MT urea
The above indicated values are rather consolidated and drastic improvements of the process do not seem to be possible.
Among the other processes with separate recycle of NH3 widely used in the past can be mentioned the ones of Toyo Engineering Ltd. indicated with the abbreviation TRC - TRC-I (Total Recycle C, TRC Improved), characterized by the fact that the ratio NH3/CO2 in the reactor is equal to approx. 4+4.5 mol and the separation of NH3 at 18+20 bar is more impressive than the previous process (Snamprogetti), also for the lack of a isobaric stripping.
Another generation of processes has been recently developed for increasing the yield in the reactor of the so called stripping processes (Snamprogetti NH3 selfstripping and Stamicarbon CO2 stripping), of the conventional values 55+65% up to 70+75%, providing at the same time, the above mentioned high yield reactors with isobaric loops.
Reference is made to the new and more recent processes as "IDR" (Isobaric Double Recycle) of Montedison and "ACES" of Toyo Engineering Ltd.
In the above mentioned recent processed "IDR" and "ACES", still operating with high ratios NH3/CO2 in the high yield reactor, all the unaltered reactants, including the high excess of NH3, are recycled to the reactor under the form of carbamate solution (the separate recycling of NH3 does not exist).
Without doubt much better yields are obtained (compared with the very first processes), but these advantages are coupled with a high complexity and complications in the construction of the equipment.
Furthermore these processes have the inconvenience of having to strip the unreacted products under high temperature conditions with real risks of decomposition of the (laboriously) developed urea.